The prior art in bottom operated unloading valves for general service tank cars is characterized by hazardous exposure of a portion of the body of the valve from below the car. This protrusion arises because flanges for adapting the valve body to the wall of the tank require outlet section breakoff grooves or shear bolts as protective devices in the attachment of the body of the valve to the tank, and valve bodies require bolted-up retaining rings or flanges of conventional design which impose severe constraints on the design considerations available for low profile tank car bottom unloading valves. Also, conventional over-center cam and link valve actuating mechanisms require high operating torque and can be over-stressed when locking up and unlocking and opening the valve, especially if the mechanism is not properly adjusted.
There is an established need for a bottom unloading valve having a low profile which can be dually-operated from either the bottom or top and does not require adjustment on assembly. Said valve should not leak under impact, have low operator working torque and less maximum stress on the valve during operation.